In one known method of mounting steam turbine blades on a rotor disk, each blade is provided at its base with a foot or a root for anchoring it to the rotor disk. The shape of the root can be likened to a Christmas tree, and the envelope of its outline forms an isosceles trapezium having its large base closer to the base of the blade, and its small base further therefrom. Starting from this envelope, the actual outline of the root includes a series of throats that may be referred to as "recesses". The portion of the outline between two recesses may be referred to as a "tang".
There may be 3 to 5 tangs on a single blade root. The rotor disk carries a series of grooves for receiving the blades. Each groove has an outline that is complementary to the "Christmas-tree" outline of the roots of the blades.
When the turbine rotates, it is these tangs, together with the complementary tangs in the grooves provided in the disk, that retain the blades. A stress field is therefore generated in the root and in the disk along the tangs.
A mean stress exists along each tang but the stress is not constant along the tang, so there exist stress peaks.